Communication devices such as terminals are also known as e.g. User Equipments (UE), mobile terminals, stations (STAs), wireless devices, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a wireless communications network, such as a Wireless Local Area Network (WLAN), or a cellular communications network sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via an access network and possibly one or more core networks, comprised within the wireless communications network.
The above communications devices may further be referred to as mobile telephones, cellular telephones, laptops, tablets or sensors with wireless capability, just to mention some further examples. The communications devices in the present context may be, for example, portable, pocket-storable, hand-held, wall-mounted, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the access network, such as a Radio Access Network (RAN), with another entity, such as an Access Point (AP), another communications device or a server.
The currently used WLAN standard, defined in the Institute of Electrical and Electronics Engineers (IEEE) 802.11ac, is based on distributed channel access through the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) technique. A communications device that wishes to transmit must first listen to a communications medium and keep on deferring from transmitting as long as the communications medium is assessed to be busy. Once the communications medium is found idle, the waiting communications device generates a random backoff period selected within a certain time range called the Contention Window (CW). If at the end of this backoff period the medium is still idle, the communications device transmits. Since multiple listeners, e.g. multiple communications devices wanting to transmit, may transmit at the same point in time after the first backoff, collisions may occur. In case of a collision, each communications device must double its contention window unless it is already at the maximum allowed value. Once a communications device gets hold of the communications channel, it may transmit its data as OFDM symbols. In addition, the Specification Framework Document (SFD) for the next generation WLAN standard, IEEE 802.11ax, envisions an Orthogonal Frequency Division Multiple Access (OFDMA) scheme to allow multiple users, e.g. multiple communications devices, to transmit and/or receive simultaneously in orthogonal subbands, called Resource Units (RU). For the purpose of uplink (UL) contention, the 802.11ax SFD describes the transmission of a special trigger frame from the AP to the communications device(-s). This trigger frame is called Trigger Frame for Random access (TF-R) and comprises information about RUs that may be randomly accessed by more than one communications device. When the communications device wants to transmit, e.g. it has a frame to send, it initializes a counter called UL-OFDMA Backoff (OBO) to a random value in the range 0 to a Contention-Window OFDMA (CWO) value. The smaller the OBO is for the communications device, the higher are its chances of winning one of the RUs designated in the trigger frames (TF-Rs) transmitted from the AP. After winning the contention, the communications device may transmit one or more management frames, such as an Association Request, a Probe Request, etc., or data packets, over one of the RUs. A particular instance of the TF-R and contention is depicted in FIG. 1. It is assumed that a first communications device STA 1, a second communications device STA 2, and a third communications device STA 3 have UL data to send and their starting OBOs are 11, 5, and 0, respectively. Upon receiving the TF-R, each of the contending communications devices STA 1, STA 2, STA 3, decrements its counter once per available RU, unless its counter has reached zero. Following this, a communications device whose counter has reached zero transmits its data on any one of the available RUs chosen at random. In the depicted instance, there are three RUs RU1, RU2, RU3 available for contention. Thus the first and second communications devices STA 1, STA 2 decrement their counters to 8 and 2, respectively. The third communications device STA 3 which was already at zero, does not decrement its counter. Following this, since only the third communications device STA3 has an OBO value equal to zero, it is the only one among the three communications devices that qualifies for transmission and therefore randomly selects an RU for its transmission. The first and second communications devices STA 1, STA 2 that failed to reach an OBO equal to zero, do not transmit in this instance, and they will wait for a subsequent TF-R where they will begin with the OBOs equal to 8 and 2, respectively.
With the introduction of Internet-of-Things (IoT), it is expected that a very large number of Long-Range Low-Power (LRLP) communications devices will be associated with an AP. The IEEE 802.11 LRLP Topic Interest Group (TIG) has recently begun working on a potential WLAN standard for LRLP communications devices. It is expected that 802.11 LRLP will be built primarily on IEEE 802.11ax. Sometimes herein the LRLP communications devices are referred to as just LRLP devices.
To decrease the collision probability in communications networks with thousands of communications devices, e.g. referred to as stations (STA)s, and thus to improve power efficiency, the IEEE 802.11ah has developed the so-called Restricted Access Window (RAW). The key idea of RAW is to limit the set of communications devices accessing the channel and to spread their access attempts over a long period of time. Essentially, the RAW divides communications devices into groups and splits the channel into slots. Then it assigns each slot to a group, and the communications devices are only allowed to transmit in their slots. This is implemented as follows. By broadcasting in beacons special RAW Parameter Set (RPS) information elements, the AP allocates one or more restricted medium access intervals, each called a RAW. During the RAW, only a set of communications devices determined according to specific rules can access the communications medium. At the beginning of the RAW, the allocated communications devices suspend and save their normal backoff function, such as values of backoff counters, retry limits, contention window, and initialize a new backoff function according to an agreed-upon Access Category (AC) which is set as RAW AC. This backoff function is used till the end of the RAW, when the normal backoff function is restored and resumed.
A drawback with the current IEEE 802.11ax contention mechanism is that it does not exploit the transmission behaviour of communications devices operating in the communications network. In addition, the RAW mechanism of the IEEE 802.11ah is limited to time-domain allocations exclusive to certain groups of communications devices.